1. Field of the Invention
Historically, small low flow rate compressors (below 800 ICFM (200 HP)) with discharge pressures below 150 psig were either screw or reciprocating type compressors. These screw and reciprocating type compressors were driven by small, induction type low speed (1800 to 3600 rpm) motors. Centrifugal compressors required high rotating speeds, this dictated use of gears to increase the drive speed to higher rotor speeds (approximately 5 to 20 times the drive speed). Initially, small centrifugal compressors were less efficient at low flow rates, had higher mechanical losses, and were not economical due to the requirement for speed increasing gears and additional bearings. Because of these problems, centrifugal compressors were not feasible for the low flow rate market segment.
2. Description of Related Art
As the centrifugal compressor technology became more efficient at these low flow rates, they were still burdened by the high cost and mechanical losses due to the speed increasing gears and additional bearings. This gave incentive to developing a direct drive high speed motor/compressor.
However, for years these speeds and power levels were not feasible. The advance in state of the art technology in power electronic components enabled the required high switching frequencies at the required power levels. But there were other problems such as being able to retain high density magnets onto a shaft that is rotating at an extreme speed. Composite fibers that have strength greater than metallic sleeves turned out to be the answer.
The purpose of this invention is to provide an improved small compact high speed motor integrally designed with compressor stages to directly drive the compressor to a desired high speed without the need of a speed increasing gear and additional bearings for the gears.
The present invention is directed to a multi-stage, such as a two-stage centrifugal compressor, driven directly by a high speed motor shaft that is integrally part of the rotor comprised of two impellers mounted directly on both ends of the motor shaft. The motor shaft which is supported on hydrodynamic bearings lubricated with oil is rotated by a stator powered by a high frequency inverter that is controlled electronically with a system that interacts with the power conversion system and Hall sensors.
In one embodiment of the invention a multi-stage centrifugal compressor with an integral high-speed motor for compressing air is comprised of:
a) a variable speed permanent magnet motor having electronically commutated stator coils surrounding a rotor carrying permanent magnets, wherein there is a gap between the coils and the rotor, said rotor having a magnetic steel rotor shaft rotatably supported at each end by pressurized oil lubricated hydrodynamic bearings, wherein the stator coils are comprised of a plurality of stator wires extending longitudinally along the stator and continuing around the ends of the stator to define end turns;
b) a plurality of compressor impellers mounted to turn with the rotor shaft,
c) a pump for introducing cooled oil into the bearings for lubricating the bearings and for cooling the bearings, rotor and stator coils, said bearing configured so that oil exiting from the bearings splashes over the supported ends of the shaft and splashes over the end turns of the stator coils, and
d) a passageway through the stator for introducing pressurized air during start up to maintain a positive pressure within the gap thereby displacing oil from the gap.
The invention is also directed to a method of starting a multi-stage centrifugal compressor with an integral high-speed motor, wherein the motor has a stator and a rotor rotationally supported therein defining a gap therebetween and wherein the rotor is supported at each end by hydrodynamic oil bearings, wherein the method is comprised of the steps of:
a) injecting pressurized air within the gap at a position between the ends of the rotor as the rotor begins to rotate to minimize the entry of oil from the bearings into the gap and
b) when the rotor reaches a rotational speed sufficient to centrifugally expel oil outwardly from the rotor ends and away from the gap, discontinuing the injection of air.
Yet another embodiment of the invention is directed to a multi-stage centrifugal compressor with integral high-speed motor comprising:
a) variable speed permanent magnet motor having electronically commutated stator coils surrounding a rotor carrying permanent magnets, said rotor having a magnetic steel shaft rotatably supported by bearings,
b) a plurality of compressor impellers mounted to turn with the rotor shaft,
c) said rotor comprising:
1) square cross-section steel shaft defined by four faces parallel to the rotating axis of the shaft,
2) permanent magnets defined by a cylindrical surface and a flat surface and having a circular segment shaped cross-section secured to each of the four faces of the square steel shaft, said magnets arranged with alternating polarity, and
3) prestressed carbon graphite fiber reinforced plastic binding for holding the permanent magnets against the shaft.
The stator and inverter are water cooled utilizing finned heat exchangers. The gap between the rotor and the stator is controlled by inducing cool air to the gap 50 that purges the area from contaminates such as oil. The rotor shaft is cooled by bathing the ends adjacent to the magnets in cool oil. An alternate or enhancement to the shaft cooling is utilizing an axial hole through the rotor from the second stage impeller to the first stage impeller. Cool air from the second stage inlet is then passed through the shaft taking heat away. The motive force for the air flow is the differential pressure from the second stage inlet to the first stage inlet.
The rotor is constructed of four half moon shaped samarium cobalt magnets mounted on a square cross sectioned steel shaft and retained by means of a prestressed composite carbon graphite fiber winding.